Abstract: An internal combustion engine with a spark timing control which varies spark timing to maintain a substantially constant peak combustion pressure timing includes a combustion pressure sensor generating a signal contaminated by high frequency noise and further apparatus which tracks the sensor signal only while it is increasing above a previous maximum and generates an output signal whenever it stops tracking to indicate the timing of a new maximum. Numbers representing the crankshaft rotational positions of the successive signals are stored in a storage element during a crankshaft rotational range in which peak combustion pressure is expected. At the end of the range, the last stored number is used to represent the timing of peak combustion pressure in the spark timing control and the apparatus reset for the next determination.